The present invention relates to communications networks.
Networks providing for the transport of encoded signals on communications links are well known. Examples are packet switching networks, voice and/or data switches, local area networks (LANS) and metropolitan area networks (MANS). The links may be ring- or bus-based and may be adapted to carry speech and/or data. Such networks allow a number of stations or nodes to communicate with each other by way of signal carrying links.
One example of such a network is an integrated switch for carrying voice and other time sensitive signals as well as data. The present applicants' European Patent Application No 85305017.7 describes a ring based network which may be adapted for switching or for use as a LAN.
For public switching applications, propagation delays must be kept short. CCITT Recommendation Q507, for example, states that the mean digital-to-digital one way propagation delay for a digital main network exchange should be 0.45 milliseconds. In the network described in the European application mentioned above, packets are assembled and placed on a ring. Each packet has a 128 bit information field. For 64 kbit/sec speech, the packet assembly time is therefore 2 milliseconds, which is unacceptably long.
There have been proposals to reduce packet assembly time by creating composite packets: see for example "Tymnet--a terminal oriented communication network", L. R. Tymes, Proc NCC Vol 38(1971), pages 211-216 and "Performance considerations for integrated voice and data networks", J. Gruber, Computer Communications, vol 4, no 3, June 1981, pages 106-126. Instead of being devoted to a single user message, the information field of a packet is shared by and number of users. For example, if sixteen 64 kbit/sec voice connections were each to provide an octet of information, packets would take 125 microseconds to assemble, which is an acceptable delay. In addition to reducing the packetisation delay, the voice load is spread more evenly over each 2 millisecond interval than when each voice connection occupies a dedicated packet. Consequently, the maximum time which may be spent waiting for access to the communications link is reduced.
In the system described by Gruber, a composite packet is assembled at an entrance node from waiting data from terminals associated with that node. The composite packet is then forwarded to an exit node, where the packet is disassembled and the data distributed to destination terminals associated with the exit node.
In such a system, if relatively few terminals at an entrance node have data for terminals at an exit node, then partly filled packets would be transmitted between the two nodes, with lower efficiency.
Another proposal for an integrated circuit/packet switching system is disclosed is U.S. Pat. No. 4,569,041 (Takeuchi et al), wherein each node includes a packet assembly/disassembly function and is arranged to assemble composite packets for respective single destination nodes. This system is similar to Gruber and results in low efficiency in use of loop bandwidth where few terminals want to communicate with terminals on a single node.